Research backs soil test recommendations

It pays to be a student of history, especially if you're a farmer, and especially if the history is more than 70 years of soil fertility research. A full fertility experiment began at north Alabama's Tennessee Valley Research & Extension Center in 1928, and farmers and researchers continue to glean important data from the long-term trials, says Charles Mitchell, agronomist with the Alabama Cooperative Extension System.

“In the early days of the agricultural experiment stations, most of the research being conducted dealt with soil fertility, but that's no longer the case,” says Mitchell.

The north Alabama experiment is called a “two-year” rotation because it always has utilized cotton rotated with another crop. “When the trial was begun, it was a cotton/corn rotation. Then, it became a cotton/cowpea rotation. Over the years, we've used different crops. But since 1982, it has been a cotton/soybean rotation,” he says.

In 1954, when the soil testing lab was established at Auburn University, researchers needed more information on continuous crop production, says Mitchell.

“That is when we started our ‘rates of N-P-K test.’ It was begun to give us a research basis for our soil test interpretations and for the recommendations that go out to farmers whenever they send in their soil samples for analysis,” he says.

Long-term trials are valuable, says Mitchell, because they allow researchers to experiment with different crops and new varieties and to evaluate their recommendations and interpretations on a continuous basis.

“These tests are at seven locations in Alabama, so we have a good basis for our recommendations that other states and private laboratories don't have,” he says.

The two-year rotation, says Mitchell, is the basis for Auburn University's cotton nitrogen recommendation of 90 pounds per acre. “That gives us near 100 percent yield. You can go a lot higher than 90 pounds per acre, but your yields won't change. If you go below 90 pounds, there's a chance your yields could drop off. In the Tennessee Valley, we reach 100 percent yield at 60 pounds of nitrogen per acre on conventional-till cotton. That recommendation was established in the 1970s and is still valid today, even with our new varieties.”

If you're growing cotton behind a good legume crop, those recommendations are no longer valid, he says.

“If you're following soybeans with cotton, you can put on no nitrogen in some years and make 100 percent of yield, and that's built into our recommendations. We say you can reduce your nitrogen rate following a good legume crop.”

Many growers want to know, says Mitchell, if they should increase their nitrogen rates if they're shooting for above-average or very high cotton yields.

“The University of Georgia does recommend increasing nitrogen rates for higher yielding cotton. We know it works on grain crops such as corn, sorghum and wheat. But it does not work on cotton. Cotton doesn't require that much nitrogen. You don't remove much N in the harvested crop, and instead of producing more lint, excess N will just produce more vegetative growth and delay maturity.”

During the past seven years, the trials in the Tennessee Valley have averaged just over 2 bales per acre, notes Mitchell, with the 60-pound nitrogen rate. In the best year, the trial averaged over 3 bales of cotton per acre.

“Again, 60 pounds of nitrogen per acre is our best rate in these Tennessee Valley soils. You can go higher, but you won't get much effect from it. In our worst year, we made one and a half bales per acre.”

Other soil fertility experiments are looking at the effect of other elements — including phosphorus, potassium and magnesium — on cotton yield, says Mitchell.

“We're looking at soil-test phosphorus versus yield. We know that at some point, you shouldn't have to put on more phosphorus. What is the point at which we no longer recommend phosphorus fertilization? Studies show that it is at 15 parts per million or 30 pounds per acre. If you're below that line, you may get only 60 percent of maximum yield in some years. Some years — even at low phosphorus — you'll get 100 percent to 110 percent of potential yield.

“But above that line, yields always are at their maximum. Our plots with no phosphorus look good, but the cotton is slightly smaller. In some years, we'll get lower yields due to low phosphorus levels.”

With the advent of precision agriculture technologies, many growers are turning to variable-rate fertilizer applications, says Mitchell. But, he adds, the results aren't always dramatic.

“Basically, the variable-rate technology has overrun the science in that we're able to precisely apply fertilizer, but we aren't as able to measure the crop responses to that fertilizer application. Soil testing is an excellent tool, but it's not always as precise as our fertilizer application methods.”

Most farmers who are using precision application methods are very good farmers, says Mitchell. “Their soil fertility levels already are high. If you look at our research — say for phosphorus and potassium — once you reach that high level, you don't expect any more responses from applied nutrients. So it doesn't matter how you put them out. You're not going to see any benefit from it. You could see some benefit from not putting it out, but we know that anyway.”

Most farmers who use precision agriculture technology don't have to put out phosphorus and potassium, says the agronomist.

“But P and K isn't the real issue with precision agriculture. There is a lot of evidence that variable nitrogen rates can pay off, and we've seen that from putting out higher rates of nitrogen where it is especially limited. Or, the yield potential might not be there due to factors such as soil compaction, lack of soil organic matter or other soil physical characteristics. In cases such as these, it might be best not to put out any additional nitrogen.

“With phosphorus and potassium applications, we can't measure a difference in the field of 10 or 20 pounds — it can't be done. We have all of this new technology, but we don't have the science to back it up.”

Mitchell recommends that growers conduct zone sampling as opposed to grid sampling in their fields. “The objective of a good soil fertility program is to build up the P and K levels. Once you reach those high levels, you don't have to put out any more, and precision application isn't going to help you.”

There is evidence, he adds, that adding lime can help in some situations. “But we're talking about big areas — maybe 2 or 3-acre blocks, and you can take care of those with a truck. You don't need the expensive equipment.”

The idea of zone management, he says, is nothing new. “Locate your differences in the field, map out those differences and add nutrients where they are needed. You don't have to invest in expensive equipment to take advantage of the technology that's available to us.”

Errors abound, says Mitchell, when you try to calibrate and show the relationship between actual yield and soil tests.

“We draw a line that we refer to as a critical soil test value, and it's subject to a lot of disagreement. It's not as precise as our ability to spread the fertilizer. We're dealing with biological systems that are highly variable.”